Manufacture of luminous electrical discharge tubes



R. R. MACHLETT wWII'kNUFACTURI') 0F LUMINOUS ELECTRICAQDISCHARGE TUBES Original Filed April 2 7, 1925 2 Sheets-Sheet 1 INVENTOR ATTORNEY Feb. 22, 1927. 1,618,767

'R. R. MACHLETT MANUFACTURE OFLUMINOUS ELECTRICAL DISCHARGE TUBES Original Filed April 1925 2 Sheets-Sheet 2 F INVENTOR M BY Z Y v3 W 4-1:

ATTORNEr Patented Pa; ,22, 1927.:

{UNITED STATES,

RAYMOND ROBER MAGHLETT, OF NEW YORK, N. Y.

MAEUFACTURE E Luminous ELECTRICAL DISCHARGE TUBES.

Continuation of application Seria1 No.'26,128, fil ed April 27, 1 925.

3326. Serial This invention relates to manufacture .of luminous electrical discharge tubes; and it relates more particularly to the manufacture of such discharge tubes which are filled 5 with a rarefied gaseous medium, such as Y neon or' some-other gas of the noble gas.

group. In luminous discharge tubes of the type here inquestion, it is-of great impor- 'tan'oe that the rarefied gaseous medium'be' .0 and remain substantially free of impurities tending to adversely affect the luminosity of said medium when the tube is in use;

\ Furthermore, it is very desirable thatthe pretsure of the gaseous medium in the tube l5 shalLremain substantially constant in order that-the conductivity of said "medium may "not change materially.

It is a principal object'of the present in- ,vention to provide a process'for the manufacture of luminous discharge tubes whereby isuch tubes can be -manufactured more easily and at less cost than heretofore, while at the -same time the characteristics of such tubes can be improved in important respects. In eneral,this object is accomplished bysub- IJeQting the tube. to a systematic preliminary treatment for elimination of occluded gase eous impuritiesfrom the walls thereof, after which pureor technically pure neon orother 0 hoble gas admitted into the previously evacuated tube and'the tube is sealed off, IlOfPlfilfiCfltlOH of the neon after introduction into the tube being necessary to render the tube suitable and readyfor use.

:5 It has been proposed heretofore to de' gasify glass tubes intended for use as luminous discharge tubes by heating to temper-4 atures "of from 360 to-4:20 for a considerable time while exhausting the tube. Much 0 of the occluded gaseous impurities can be I eliminated from the glass in this way, but

-.the method is extremely slow and requires the very bestof equipment. Even under the' most favorable conditions it is pracl5 .tically impossible by such prior methods to carry the de-gasification tosuch a'point as 'to preventgradual depreciation of the finished device through contamination of the contained gaseous medium from subsequent evolution 'of small quantities of gas PAT NT (OFFICE.

This application filed September 24, No. 137,548.

eous impurities from the continued operation. 5; X I

According tothe method of the present invention, the glass tube is so processed during the exhausting operation andsbefore' introduction of the rarefied gaseous medium 7 that, after sealing and in subsequent operation and use, no appreciable amount of impurities is evolved from the glass. This result is accomplished with the-aid'ofa purging agent (employed in vapor form in a manner to be hereinafter described As ex:

glass walls during .amples of such purging agents may be mentioned the alkali metals which have a strong chemical aflinity'for precisely such-gases as are undesirable in the form of impurities. in the luminous gaseous medium contained in the finished discharge tube or lamp and which, inthe present process, are made to combine with such gases chemicallyand to aid in their removal from thetube. ,Purging agents of this general character have been used heretofore in the art but not, so far as I am aware, in the manner to be hereinafter set forth.

In carrying my invention into effect-, I preferably make use of a metallic potassium as the alkali metal, the vapor of which is employed as a purging agent as will be hereafter described. This metal. is preferred since its temperature of volatilization is conveniently low and hence it becomes possible to maintain the vapor in the tube without condensation at temperatures which are well below the point at which the glass walls of the tube may soften, particularly if the tube is made of high-melting point borosiliga glass, as will be later explained. It

will be understood however that other alkali metals may be employed, provided the tubes are composed of material which will permit the employment of temperatures sutficiently high as 'to prevent an objectionable degree of condensat on of the vapor. a I find that when an-alkali metal vapor is used as a purging agent within .the tube, as will be explained, apparently the gaseous impurities are removed from the glass walls in excess of five thousand hours..

f the tube to such a depth, or that such fication or scouring of the tube walls is" much more effective than could be secure of the tube walls.

by heat alone even if temperatures were used above the melting point of the glass. seems to be a fact that by treating the tube in the manner hereafter explained with an alkaline vapor metal a chemical change occurs to a very considerable extent in the glass wall. The latter opinionis based upon the observed fact that glass after prolonged treatment can be made to combine only with difiiculty with normal glass of the same initial composition.

It is by no means certain that the co'htamination to which the conducting gaseous medium may be subjected is due to the presence in the glass walls of occluded aseous impurities and it is robable that 1; 1e contamination which has ieretofore been encountered and which lowers the life of the tube may be due to the decomposition of salts entering into the composition ofthe glass and which have a definite vapor pressure, 'such decomposition being the result of the electrical strain and instantaneous temperature conditions accompan ing ionic bombardment y subjecting the tube to the treatment with vapor of the alkaline metal as hereafter described, the vapor pressures of such salts are apparently re-' duced to a point where ionic; bombardment no longer can affect decomposition or the liberation of impurities.

It is furthermore an important feature of the invention, in its more specific aspects, to construct the tube of high-melting-point boro-silica glass. This is for the reason that ordinary lead or flint glass is easily discolored and otherwise adversely affected when it is heated beyond certain critical values in the presence of alkali metal vapors. With hi h melting point boro-silica glass this disco oring may be entirely eliminated even though the temperature be very considerable, say of the order of 350-400 C. whlle in contact with alkali metal vapor, which high er temperature is desirable in view of the fact thatthe rate of diffusion of the. vapor through the tube is a function of temperature and by the use of such high temperature the time of processing may be considerably shortened and longer tubes may more easily be formed. r

The use of high-melting point boro-silica or similar glass forthe construction of luminous tubes is furthermore advantageous in view of the fact that glass of this character has great dielectric strength, a very impor- This puri-Q that t shown the tube evenly to 600 oven being taut consideration when exterior electrodes are made use of and where the discharge within the tube is due to a' capacity effect. The present invention finds its greatest value with tubes employing exterior electrodes for the reason that when such electrodes are used very litle energy is developed within the tube and' therefore the most favorable internal conditions are essential. Because of this fact, the presence of impurities in thegaseous conducting medium or variations in pressure of the conductingmedium manifest themselves as deteriorating influences of much greater relativemag- I nitude than when interior electrodes are used, with the consequent possibility of developing much greater energywithin the tube.

It has been found in fact that tubes may be i made with exterior electrodes underthe resent invention where the-maintenance o the desired conditions of compgsition and pres-. sure-of the gaseous medium may be so uniform as to give a life'of from six thousand to ten thousand hours of continuous operation.

At the same time the invention is distinctly advantageous in connection with tubes employing internal electrodes ,for the reason e process provides a way by which gaseous impurities may be rapidly and ef ectively removed from the walls of the'tube and electrodes, thus permittin a tube after manufacture to be immediately used without the necessity of preliminary seasoning. Therefore,. while the present invention is very specially advantageous in connection with tubes employing exterior electrodes and will be so claimed, yet certain of the claims hereafter to be made are so worded as to include interior electrodes as well.

The. principles of the invention will be more fully understood from the followingzdescription of a practical embodiment of the 7 method or process of the invention, wherein reference will be made to typical apparatus in the accompanying drawings, wherein-,-

Fig. 1 shows diagrammatically such an apparatus system suitablefor use in carrying out the process;

Fig. 2 shows a finished lamp tube with ex-- terior electrodes and an therefor and electrode for such lamp tube.

Referring to the drawings, and more par-' ticularly to Fig. 1, 10 is an oven large to contain the whole lamp or tube 11 and capable of raising the temperature of C. or higher, said provided with suitable heating means, not s own. The tube '11 is most'de- "six-ably of high-melting point boro-silica glass and. is shown as being provided with external terminal electrodes each of which operating circuit Fig. 3 shows a modified form of external 7 l sufiiciently in this instance consists of a silver coating. These electrodes'willbe described in greater detail presently in cennection with Fig. 2. To one end otthe' tube 11 is sealed an extension 12; connectingthe tube with a small container 13 which is located outside the oven shown.

and contains an appropriatepurging agent j which in the present example 18 pure metallic potassium. To theother end of the lamp tube is sealed aglass tube 14 by means of which For testing "for the presence ofgas, a

I spectrum tube 21 is connected with the system and supplied with high potential current at its terminals from an appropriate source 22,

a valve 23 being rovided for disconnecting the tube 21 from t 1e pump 18 during testing. Durin the evacuation of the tube the test is made by closing-thevalve23 and-applying the proper potential to the terminals of the spectrum :tube 21while continuing the operation of the difiusion pump 15. The withdrawaLof even very slight quantities of gas from the lamp tube 11 by'the diffusion pump will in a few minutes time give. a

pronounced indication in the spectrum tube 21,- thus providing a very. delicate test. At one stage in the processing of the tubes, an

electrical discharge is passed through the tube 11, and forthis purpose a source 2; of

high potential high frequency current is provided and arranged to apply potential to theterminals of the tube when desired through the connections indicated. A

The method of processing the tube 11 and forming the complete lamp may be carried out in a typical instance as'follows:

. The tube which is assumed in this instance to be of high-melting'boro-silica glass, is

-first throughly bakedby heating it in oven '10 for a-substantial period Oftiine, say

or minute's, at a relatively high temperature which should be above 300, C. and which, ina typical instance, may approximate 500 0. During this baking treatment, the high vacuum system is maintained in operation to dislodge most of! theocc'lud'ed gases and volatile material from the glass and to evacate the tube. This results in de-' asifying the glass to a point where most of the occluded gaseousimpurities are removed. In order to substantially complete the de-gasification and also to ensure that a there will "be" no, further substantial; evolu-' tionof contaminating gases .-from the" glass after, the finished lamp tube is operating in 7 actual service, the tube is newt-subjected to a further de-gasifying and stabilizing treatment which constitutes an.important tea tureof the invention in its best embodiments. lVit-h the high vacuum system still in operation, the'heaiting of the oven is discontin-' 11ed, the oven is opened and the tube is allowed to cool down considerably. now applied to the container bulb 13 and the Heat is metallic potassium is distilled over into the extension 12 inside of the oven but'not as yet into the lamp tube 11'. The container 13 and such portion of the tube 12 as extends outside the oven are noiv sealed ofl',-the oven is closed again and heating is resumed until.

the tube is brought to a temperature of about 350 C. The potassium metal in the tube' 12, being vaporized, diffuses into the lamp tube 11'. The lamp tube is now excited by application of high frequency potential applied to its external term nal electrodes a-a fronithe' high frequency supply source 24:. A high frequency current discharge is thus caused to pass through the tube while the alkali metal vapor permeates it. This causes a band of yellowish light, preceded by a short band of purplishlig'ht to diffuse through the tube from the end nearestextension 12 to the opposite end. Excitation of the tube under the conditions specified is continued until and for some minutes after-thetube isfilled with the yellowishlight aforesaid, after which the electrical dischargeis discontinued, the high vacuum system disconnected from the lamp tube and' the lamp tube' cooled. Neon gas of the requisitepurity-is now admitted into the dega'sified; and highly evacuated lamp tube until the proper pressure inthe tube is attainecl. Inpractice t-his pressure inay vary from 3 to 8 mms. of nlercury absolute, or thereabouts, an absolute pressure of about 14 mms. being usual. Without further treatment', the .tube v10 is then sealed off at both ends, beingnow complete and ready for use.

The-term ffpure neon as employed herein signifies n'eon-fofsuch purity that in a tube of the type hereinabove described it will have sufficient conductivity and brilliancy to be commercially practical;- For eXaInple, a

commercially purified neon known as neonhelium spectralrein, which is meon that has been spectroscopically tested and found to contain practically no impurities asidefrom helium, serves admirably .inthe practice of .the present invention N -,eon gas of this technical purity is obtainable on the market as a commercial product.

Instead of using the additional vessel 13 containing the i 4 purifying metal, other methods of obtaining a discharge of alkali metal through' the tube may, be.utilized.-

or example, it is possible to heat certain areas of the glass walls of the lalnp'tubc, by

' the tube.

placing heating caps over such area, to a temperature at which it becomes electrically con uctive, whereupon a certain amount of sodium may be electrolyzed out of the glass by applying a direct potential of suitable voltage and polarity between these heated areas. If the tube is maintained at a temperature so high that the metal is vaporized to some extent and not recondensed within the tube, the 'alkali metal vapor will be caused to diffuse through the tube and thus serve the purpose of the potassium vapor in the above described method of processing able absorption of neon. The conductivityof the rarefied gaseous medium would thereby be lessened and the luminosity or brilliancy thereof would also be adversely affected.

A typical form of the finished luminous discharge tube is shown in Fig. 2. where A indicates the enlarged end portions having the external coatings aa of silveror other easily deposited metal, which constitute the external electrodes. In the form illustrated the terminal or electrode portions A-A are shown larger or of greater diameter than the intermediate portion, but this is not essential to the invention in its broader aspect, their size being governed to a large extent by the frequencyof the current used, the important point being that the relation be snchthat, in operation, the impedance of the gas in the tube will be greater than that of the electrode system. Thatis, the distribution of impedance in the system as a whole, should be such that the-energy loss occurs chiefly in the gas in the tube and relatively little in the glass electrode sys 'tem. In achieving this result, operating frequencies of the order of 750,000 cycles The connected with per second can be used with good results. use of frequencies of thisorder permits the use of relatively small external electrodes which makes such frequencies especially desirable.

In the operatin shown in Fig, 2, leads external electrodes a-a of the tube with the secondary windings 27 and 28 of Tesla coils, of which the primaries indicated at 29 and 30 are oppositely wound. The'windings 29and 39 are included in an oscillating circuit actuated by the circuit breaker 31, a suitable current source circuit arrangement through binding posts 33, the oscillator cir- 25 and 26 connect the cuit comprising the windings 29 and 30 and the condenser 32' connected as shown. Other circuit arrangements may be used for accomplishing the same result but that here shown is simple'and convenient. In constructing the oscillator, the usual precautions necessary to guard against losses in high frequency circuits should be observed.

In Fig. 3 is shown a modified form of external electrode which may be used at each end of the lamp tube instead of those shown in Fig. 2. The electrode as a whole is indicated at A, and comprises a chamber 33 formed by inner and outer walls 34 and 35, formed on the lamp tube, said chamber, 33 being filled with graphite or other conductive material in powdered form, lead Y sealed in position at 36, being provided for application of operating potential.

It will be obvious that if interior electrodes are used, they may be of any usual construction, the discharge between them being effected by sources of electrical potential with which the art is familiar. When such electrodes are used, the processing of the tube will be carried out as above explained resulting not only in the elimination of gases or other impurities also from the walls of the electrodes.

As is well known in the manufacture of luminous tubes with a gaseous conducting medium of noon and with interior electrodes, it is the practice to subject the tubes both during and after exhaustion to the effect of a current, which durin the exhausting operation is of considenhly greater magnitude than is employed under working conditions, this seasoning being continued for a number of hours. In this way, the gaseous impurities in the neon are gradually absorbed by the metal film formed on from the tube walls, but

the tube walls by vaporization of the elec-' trodes. By subjecting thetube to the initial processing as I have explained, the purification is so complete as to make any seasoning of the tube. unnecessar and therefore neon tubes with internal e ectrodes constructed in accordance with my present invention may be put into use immediately after manufacture. F In the foregoing description, reference has been made more particularly to the employment of neon as a rarefied medium em; ployed in the lamp tube. It is to be understood, however, that the invention is not restricted to .the use of neon but" extends also to the manufacture of discharge tubes of the general type mentioned whether neon be employed as the rarefied gaseous medium or some other gas. of the noble gas groupsuch as helium, krypton, xenon, argon, or the like, or anycombination of these gases; or neon or any other suitable gas, together with the vapors of metals such as mercury or iodine or other. combinations for the pursubstances capable of impairing the operpose of obtaining various color e'fl'ects.

ThISELPPllCatlOD is a'continuatio'nof ap ph antscopending application Serial No.

26,128, filed-April 27, 1925. 1 What I. claim is: t

\ 1. The method of making a luminous electrical discharge tube, containing a conducting gas of the noble gas group, which comprises purging the walls of the tube by means of an agent. adapted to. coact with substancescapable of absorbing or contaminating the gas during operation of the tube to an extent sufficient to/c-ommerciafiy impair the color or amount of light produced,

. and thereafter filling the tube with the conducting gas at low pressure and sealing the same...

making a luminous elecin which the purging agent is an alkali metal vapor.

3. The method of making a luminous electrical discharge tube which, comprises purging the walls of the tube of undesirable substances by causing said substances to coact with an agent to such an extent that thespectrum of the initial gas fillingin dicates' the absence of impurities.

4. In the manufacture of luminous electrical discharge tubes of glass containing a conducting gas of the noble gas group, the method of'purging the walls of the tube of substances capable of.' impairing the operating qualities of the gas, which comprises heating the tube and, while maintaining'the same at' a high temperature, evacu ating the same, diffusing an alkali metal ,va- 4015M into the tube which vapor does not conense at the temperature-used, maintaining an .-elect rical discharge therethrough, and continuing the heating until the alkali metal is remove 5. The method of making luminous electrical discharge tubes of glass containing a conducting gas of the noble group which comprises purging the walls of the tube of I substances capable of impairing the operat ing qualities of the. gas, by heating the tube to atemperature between 300 and 500 C. while maintaining evacuation-of the tube,

distilling into" and diffusing therethrough an alkali metal, the vapor of which does not condense at the temperature used, and maintaining an-electrical discharge therethrough continuing the heating until the alkali metal is substantially all removed,- then discontinuing the electrical discharge, cooling thetube, filling thesame with the desired gas at low pressure previously purified to a de-. -gree suifi'cient'to give the operating qualities group, the method of purging the tube of atingqualities of the gas to be used in the tube, wh ch comprises heating the tube and maintaining 1t at a temperature of the orderof 300 degrees centigrade,-while diffusing the vapor of an alkali metal through the tube, exhausting the tube of the metal vapor and other ivapors and. gases formed, and maintaining an elect-rlcal discharge through the tube by way of terminal connections out I ofphysical contact with the contents of the tube, then discontinuing the electrical discharge, cooling the tube, admitting. the conducting gas intothe tube at a low pressure,

and sealing off the tube.

f 7. In the manufacture of luminous dis-' charge tubes of glass of the boro-silica type containing a conducting gas of the noble gas group, the method of purging the tube of while evacuating the same and continuing this treatment until the degasification of. the tube is carried to a stage where most of the impurities are removed, then introducing potassium vapor into the tube and passing an electrical discharge through the tube by means of electrodes out of physical contact discharge andheating until and some time after the tube is vfilled with light, then discontinuing the discharge, cooling the tube, admitting the gas to be used as the luminous medium, and se'aling ofl'the tube.

with the interior of the tube, continuing the 8. In the manufacture of vacuum tubes, I

the step of urging a tube of impurities which comprises passing an electrical dis--- charge through said tube while it is in heated condition and contains a purging agent in vapor form at low pressure, the temperature of the tube being sufficient" to prevent condensation of the vapor.

9. In the manufacture of vacuum tubes,

the step of purging a tube of impuritieswhichcomprises passing a high frequency electrical discharge through said tube while it is in heated condition and contains po- ,tassium vapor at low pressure, the temperature of the tube being sufiicient to pre vent condensation of said vapor.

10. An electrical discharge device for illuminating purposes comprising a glass tube of which the interior is free of substances capable of absorbing or contaminating a conducting gas in quantity sufficient to commercially impair the color or amount of: light produced by such gas when energlzed by passage of an electrical discharge therethrough, said tube containing a conducting gas sealed therein and under low pressure, and a pair of external electrodes for effecting electrical connection with the gas content by capacity coupling. a 11. An electrical discharge tube of the character described composed of higl1-1nelting bbro-silica glass, and containing a luminescent aseous'medium at low pressure,

and being iree of discoloration, the interior of said tube being free of substances capable of absorbing o1 contaminating a conducting gas such as neon in quantity suflicient to commercially im air the color or amount of light produced y such gas when energized by passage of an electrical discharge 1 therethrough.

RAYMOND ROBERT MACHLETT. 

